Mouse fibroblasts growing in vitro are inhibited by glucocorticoids. The cells possess a macromolecule receptor which specifically binds glucocorticoids with high affinity. This study examines the biological and biophysical properties of this receptor macromolecule, and the relationship of the receptor to the phenomenon of steroid hormone resistance. Glucocorticoid-resistant variants of the fibroblast are grossly deficient in the receptor macromolecule. Information will be developed on the intracellular translocations of the receptor macromolecule. Unbound receptor is found in the soluble portion of the cytosol. Upon addition of steroid, binding takes place followed by a temperature dependent translocation into the nucleus. Within the nucleus, it exists in two forms. The major portion is readily extracted by 0.3 M KC1 (the "nuclear extractable" form). A small part is tightly bound to chromatin ("nuclear residual" form). Energy deprivation (KCN treatment, glucose deprivation) drives most of the nuclear receptor into the residual form. The residual form can be isolated by treating chromatin samples with DNase. The laboratory is currently studying the physicochemical properties (molecular weight, sedimentation rate, axial ratio, isoelectric points) of the three forms of steroid-bound receptor. Available evidence indicates that the receptor macromolecule undergoes a marked allosteric alteration subsequent to binding the glucocorticoid. The change in shape is dependent on temperature and ionic strength of the solvent. We are also preparing spin-labeled analogs of glucocorticoid hormones and hope to examine the receptor-steroid interaction by ESR techniques. The effects of steroid treatment on histone synthesis will also be studied.